A colleague recommended API Monitor from rohitab.com for monitoring COM calls between components. I’ve installed it and have to agree that it looks really promising:
For the tool to be useful, you have to tell it about your custom COM interfaces by generating an XML file in a similar format to IDL – obviously room for improvement there by the authors (I wonder why they don’t read IDL files natively?). Having said that, once you have provided the interfaces, API Monitor will log all calls via those interfaces on any components that you specify. Better still, you can inspect the values in each call and even set breakpoints that will take you into the debugger of your choice (e.g. Visual Studio).